This invention is based on electroviscous suspensions containing more than 25% by weight of silica gel with a water content of 1 to 15% by weight as disperse phase and liquid hydrocarbons as liquid phase and a dispersing agent.
Electroviscous fluids (EVF) are dispersions of finely divided hydrophilic solids in hydrophobic, electrically non-conductive oils whose viscosity can be increased very rapidly and reversibly under the influence of a sufficiently powerful electric field from the liquid to the plastic or the solid state. Both continuous and alternating electric fields may be used for altering the viscosity. The currents flowing through the EVF are extremely low. EVFs may therefore be used wherever powerful forces are required to be transmitted by means of a low electric output, e.g., in clutches, hydraulic valves, shock absorbers, vibrators or apparatus for positioning and fixing workpieces.
The general requirements in practice are that the EVF should be liquid over a temperature range from about -50.degree. C. to 150.degree. C. and chemically resistant and should show a sufficient electric viscous effect, at least in the temperature range of -30.degree. C. to 110.degree. C. It is also important that the EVF remain stable over a long period of time, i.e., no phase separation should take place and in particular there should be no formation of a sediment which is difficult to redisperse.
EVFs based on silica gel dispersions in non-conductive oils have been fully described in British Pat. No. 1 076 754. In these dispersions, water-containing silica gel particles are dispersed in a non-conductive oil. The water content of the silica gel particles and the form in which this water is bound should be critical with respect to the electroactivity of the EVF. Non-ionic surface active agents and/or surface active agents containing a basic nitrogen atom are used for dispersing the particles. These EVFs, however, have a strong tendency to settle and to form a sediment which is difficult to redisperse, and the examples given in the abovementioned Patent also relate to highly thixotropic fluids and pastes. Both these properties, however, restrict the possible applications of electroviscous fluids. This applies particularly to the use of a thixotropic EVF as coupling fluid or the use of a paste as hydraulic fluid.
It is at this point that the invention sets in.